This invention relates generally to xerographic reproducing apparatus and more particularly, to a toner fusing apparatus therefor.
In the process of xerography, a light image of an original to be copied is to be recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual image can be affixed directly upon the photosensitive member of transferred from the member to a sheet of plain paper or other suitable copy substrate with subsequent fixing of the image thereto.
In order to permanently affix or fuse electroscopic toner material onto a support member, it is necessary to treat the toner material, for example, by the application of heat or a solvent for the toner material which causes the constituents of the toner material to coalesce and become tacky. Stated differently, the application of heat or solvent must increase the flowability of the toner material whereby it is absorbed to some extent into the fibers of the support member which, in many instances, constitute plain paper. Thereafter, as the toner material cools or the solvent vapors are driven off, solidification of the toner material occurs causing the toner material to be firmly bonded to support member.
The most common method of fusing toner images employs the combination of heat and pressure. However, the power requirements for such systems are so great that they do not lend themselves to long life and energy conservation which directly bear on the overall costs of the fuser sub-assembly.
In a pure solvent fusing apparatus utilized in the higher speed machines, the quantity of solvent required to effect fusing of toner images is sufficiently high so as to present problems of toxicity and economical solidification of toner images within a reasonable time frame.
It has been demonstrated by various persons working in the xerographic arts that toner images can be fused by exposing the images to solvent vapors or liquid solvents, the ratio by weight of solvent to toner for fusing to take place being at least equal to 0.20 and not greater than 1.0 at room temperature. The ratio is determinative of the speed at which fusing takes place (i. e. the higher the ratio the faster the fusing at any given vapor pressure of the solvent).
It will be appreciated by those skilled in the art that it is desirable to fuse toner images using as little solvent (i. e. minimizing toxicity and cost problems) as possible and as fast (i. e. meeting consumer needs or requirements) as possible. One method of accelerating the fusing rate for a predetermined solvent to toner ratio would be to reduce the paper temperature relative to the temperature of the solvent thereby increasing the rate of fusing by increasing the rate of condensation of solvent vapor. However, it is believed that this mode of accelerating fusing has adverse effects on the copy background and it is not considered an acceptable technique for such purposes.
The solubility of known solvents in typical (i. e. 41% and - butylmethracalate and approximately 59% polystyrene or a combination of polyethylene and carbon black) toners is such that at room temperature fusing is very slow and at 30.degree. to 40.degree. F. fusing does not take place at all. The foregoing is believed to be due to insufficient amounts of solvent which toner will absorb at these temperatures. Accordingly, increasing the temperature of the toner and solvent simultaneously accelerates fusing but such acceleration is limited by the boiling point of the solvent and rapidly decelerates at higher temperatures by reducing the partial pressure of the solvent.
Accordingly, it is a primary object of this invention to provide a new and improved fuser for use in electrostatographic copying apparatus.
It is a more particular object of this invention to provide an improved fuser for fixing toner images to copy substrates which fuser utilizes a solvent for the toner.
Another object of this invention is to provide a solvent fuser which permits faster fixing at relatively low solvent to toner ratios.
Still another object of this invention is to provide a solvent vapor fuser which effects faster fusing of toner images at a predetermined solvent to toner ratio.